JP2001288509A - Method for producing steel material excellent in toughness at welded joint part - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve HAZ(heat-affected zone) toughness by enabling the fine dispersion of multiple oxide and the increase of the multiple oxide pieces, and stably dispersing the oxide which can realize excellent HAZ toughness through the fining of austenitic grain and the development of fine ferritic grain while improving the upper limit of A1 content to the same degree as the ordinary Al-killed steel. SOLUTION: In a deoxidizing process, dissolved oxygen quantity in molten steel is adjusted, the deoxidation is performed in the order of Ti, Al and Ca, and further, Al is added, thus Ti-Al-Ca oxides having 0.005-2.0 μm grain diameter is obtained and 100-3,000 pieces of such grains are uniformly and finely dispersed to lastly produce a steel material for welded structure excellent in the toughness in the base material and the welded metal.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a welding heat affected zone (hereinafter referred to as H) used for ships, offshore structures, middle and high rise buildings and the like.
AZ) and a method for producing a steel material for a welded structure having excellent toughness.

[0002]

2. Description of the Related Art In recent years, the demand for material properties of welding steel materials used for large structures used in ships, marine structures, middle and high-rise buildings and the like has been increasing strictly. The requirements for the toughness of the HAZ are also increasing.

Further, when constructing such a structure, it is desired to apply a large heat input welding method typified by a flux-copper backing welding method, an electrogas arc welding method, etc. in order to promote the efficiency of welding. Have been.

[0004] In response to this, HAZ of steel material during large heat input welding
There have been many proposals focusing on toughness, and recently, techniques utilizing fine particles mainly composed of oxides in steel have been actively developed.

[0005] For example, in the field of thick plates,
No. 5, JP-A-62-103344, JP-A-6-103344
As exemplified in JP-A-2-214126,
There is Ti deoxidized steel containing Ti oxide. These techniques mainly utilize Ti oxides as HAZ intragranular ferrite generation sites. However, it is difficult to control the number and dispersity of Ti oxides in steel simply by adding Ti to molten steel. In steels in which Ti oxides are dispersed only by Ti deoxidation, for example, Problems such as insufficient number of Ti oxides and variation in toughness in the thickness direction of a thick plate are observed. The cause is coarsening and aggregation of Ti oxides. If the number of Ti oxides is to be increased, coarse T oxides of 5 μm or more are required.
i oxides, so-called inclusions, increase. This 5μ
Inclusions of m or more are harmful as starting points for structural destruction and cause a decrease in toughness. Therefore, additional H
In order to achieve an improvement in AZ toughness, it is necessary to utilize an oxide which is less likely to be coarsened and aggregated and which is more finely dispersed than a Ti oxide.

Further, in the method disclosed in Japanese Patent Application Laid-Open No. Sho 61-79745, the upper limit of the amount of Al is limited to a very small amount of 0.005% in order to easily form a Ti oxide. . If the amount of Al in the steel is small,
There may be a case where the toughness of the base material is reduced due to a cause such as an insufficient amount of N precipitates. Further, when a steel sheet having a small amount of Al is welded using a commonly used welding material, the toughness of the weld metal may be reduced.

To solve such a problem, some of the inventors have disclosed in Japanese Patent Application Laid-Open Nos. 6-293937 and 9-359.
No. 8 discloses a technique for utilizing the generated Ti-Al composite oxide by adding Al immediately after adding Ti. This technology is based on the idea that in the deoxidation process in steelmaking, deoxidation is performed while gradually decreasing the dissolved oxygen in the molten steel in a stepwise manner. For this purpose, deoxidation proceeds in the order of Ti, Al and the weaker deoxidizing power. That is the point. This makes it possible to disperse oxides more finely than Ti deoxidized steel, and to greatly improve large heat input welding HAZ toughness while increasing the amount of Al. In the industry, a further increase in welding heat input of 200 kJ / cm or more has been promoted, and HA has been further increased.
A steel material having Z toughness is required. Also in this technique, the upper limit of the amount of Al is set to 0.02%.
The amount of Al is still lower than that of general Al-killed steel, and the versatility of welding materials has not yet been completely overcome.

[0008]

Since the HAZ characteristics can be further improved as compared with the above-mentioned conventional method, fine dispersion and increase in the number of oxides can be achieved, and excellent austenite grain refinement and fine ferrite formation are achieved. An object of the present invention is to stably disperse an oxide capable of realizing HAZ toughness, and at the same time, further improve HAZ toughness while increasing the upper limit of the Al content to the same level as that of a general Al-killed steel.

[0009]

Means for Solving the Problems The present invention has been made to solve the above-mentioned problems. Means 1 is to deoxidize by adding Ti to molten steel having a dissolved oxygen concentration of 20 to 80 ppm. After the sol. Al is 0.004 ~
Al was added so as to have a concentration of 0.02%, and then Ca was added. Then, Al was further added, and C: 0.0
3 to 0.18%, Si: ≤ 0.5%, Mn: 0.4 to
2.0%, P: ≦ 0.02%, S: 0.001 to 0.0
1%, sol. Al: 0.005 to 0.09%, Ti:
0.005 to 0.02%, Ca: 0.0005 to 0.0
04%, N: 0.001 to 0.006%, the remainder is molten steel composed of Fe and unavoidable impurities, and the molten steel is cast in a continuous casting process to have a particle diameter of 0.005 to 2.0.
[mu] m, is a method for producing a welded joint toughness steel material superior in the composite oxide containing 100 to 3000 pieces / mm ² containing Ca, Ti, and any two or more of Al as the composition.

[0010] The means 2 may be composed of Cu: ≦ 1.
0%, Ni: ≤ 1.5%, Nb: ≤ 0.03%, V: ≤
0.1%, Cr: ≦ 0.6%, Mo: ≦ 0.6%, B:
The method for producing a steel material excellent in toughness of a welded joint portion according to the above means 1, wherein one or more of 0.0002 to 0.002% is contained.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail.

The inventors of the present invention have made use of oxides to simultaneously reduce the size of austenite and generate intragranular ferrite in a region heated to 1400 ° C. or more in the vicinity of a weld bond, as metal structure factors for improving HAZ toughness. Considered to achieve.

In order to reduce the size of austenite, it is necessary to suppress the growth of austenite grains at high temperatures. As a means for this, a method of pinning an austenite grain boundary with an oxide and stopping the movement of the grain boundary can be considered. Therefore, in order to refine austenite, it is effective to generate a large number of fine oxides.
From such a viewpoint, when the oxide present at the austenite grain boundary was observed in detail, the particle diameter was 0.005.
１．０1.0 μm was found to be major. That is,
The presence of the oxide having a particle diameter of 0.005 to 1.0 μm in the steel enables the pinned austenite grains to be refined.

Regarding the formation of intragranular ferrite, the present inventors observed the structure of intragranular ferrite formed in austenite grains and investigated the particles contained in the intragranular ferrite. As a result, 0.1 g of nuclei of intragranular ferrite was
A composite of Ti-Al-Ca oxide having a size of ~ 2.0 m and containing any two or more of Ti, Al, and Ca, and a Ti nitride + MnS deposited thereon works effectively. I found something. The oxide is stable even when heated to a high temperature, and exists stably in steel without change even at 1400 ° C. or higher. Also, Ti nitride + Mn
S precipitates a Ti—Al—Ca oxide as a nucleation site in the subsequent cooling process, so that intragranular ferrite can be generated in the vicinity of the weld bond.

From the above findings, it is necessary to reduce the size of austenite grains in a region heated to less than 1400 ° C. and to produce intragranular ferrite in a region heated to 1400 ° C. or more near a weld bond portion. Diameter 0.01 ~
It is necessary that 2.0 μm of the Ti—Al—Ca composite oxide be present in the steel. According to the findings of the present inventors, when the particle diameter is less than 0.01 μm, the effect as a Ti nitride precipitation nucleus is weak, and when the particle diameter exceeds 2.0 μm, there is a possibility that the oxide may become a starting point of fracture. And the HAZ toughness may decrease.

Next, the number of Ti—Al—Ca oxides will be described.

FIG. 1 shows the number of Ti—Al—Ca oxides and H
The relationship with AZ toughness was shown. If the number of oxides is too small, sufficient grain boundary pinning and generation of intragranular ferrite cannot be obtained at the time of welding, so it is necessary to have 100 or more oxides / mm ² . As the number of oxides increases, the number of Ti nitrides and intragranular ferrites increases and the HAZ toughness improves, but if there is an excess of oxides exceeding 3000 / mm ² , the toughness of the HAZ portion and the base material is reduced. Therefore, the upper limit of the number of oxides is 3
000 pieces / mm ² .

The size and the number of the oxide are measured in the following manner. An extract replica is prepared from a steel sheet as a base material, and the size and the number of the oxide are measured by observing at least 20 visual fields at a magnification of 10000 and an observation area of 1000 μm ² or more with an electron microscope. At this time, an extracted replica collected from any part from the surface part to the center part of the steel sheet may be used.

Hereinafter, the production method of the present invention will be described in detail. First, the present inventors carried out deoxidation experiments in various orders using various deoxidizing elements in order to effectively and uniformly disperse a large number of precipitates such as Ti-Al-Ca oxide and TiN and MnS. Tried. As a result, when performing deoxidation treatment,
It has been found that it is effective to sequentially deoxidize an element having a weak deoxidizing power to an element having a strong deoxidizing power in order to finely disperse a large number of oxides. This is because the deoxidation reaction is repeated while keeping the degree of supersaturation with the dissolved oxygen in the molten steel small, so that rapid growth and coarsening of the oxide are suppressed. When Ti, Al, and Ca were used as deoxidizing elements, Ti having a final content of 0.005 to 0.02% was added to molten steel whose dissolved oxygen concentration was adjusted to 20 to 80 ppm, and deoxidized. After that, immediately after the sol. Al is 0.004-0.
The most common method is to add Al so as to be 02% and then to add Ca so that 0.0005 to 0.004%. Ti-Al-Ca oxide and TiN, Mn
Precipitates such as S were uniformly and finely dispersed, and when the obtained steel material was subjected to large heat input welding, a result was obtained in which the toughness of the HAZ portion was extremely excellent in welded structural steel.

With respect to the dissolved oxygen concentration before the introduction of Ti, if the dissolved oxygen concentration is less than 20 ppm, an amount of oxide required to secure HAZ toughness is not formed, while
If the dissolved oxygen concentration exceeds 80 ppm, a large number of coarsened oxides are generated, and these become the starting points of brittle fracture, thereby causing a decrease in HAZ toughness.

Sol. For the amount of Al, sol. If the amount of Al is less than 0.005%, the amount of dissolved oxygen is not sufficiently reduced, the effect of making the oxide finer is reduced, and the oxide becomes coarse and floats. On the other hand, if the content exceeds 0.02%, the previously generated Ti oxide is completely reduced, and the oxide becomes alumina. However, alumina easily aggregates and coalesces, and the oxide becomes coarse and floats. This leads to a reduction in the number.

Next, by further adding Ca, which has a stronger deoxidizing power than Ti and Al, the oxide that has already been produced is partially reduced to Ti-Al-Ca oxide. Further, the dissolved oxygen concentration further decreases, and Ti-Al-
The growth of Ca oxide is further suppressed, and the oxide can be dispersed while keeping the fineness. At this time, excessive addition of Ca causes lowering of the melting point and coarsening of the oxide,
It is not appropriate because it promotes the formation of aS and inhibits subsequent MnS precipitation.

Regarding the timing of deoxidation, a molten steel sample after Ti deoxidation was appropriately collected, and the formation behavior of oxides was examined. As a result, the Ti oxide generated with the lapse of time after Ti deoxidation grew and aggregated. It was clarified that the particles became coarse and surfaced. Therefore, it is effective to introduce Al immediately after mixing Ti into the molten steel uniformly after the introduction of Ti in order to obtain a large amount of oxides. Therefore, Al
It must be added and added in a deoxidation step in a secondary refining facility such as RH where Ti is added. However, when Ti deoxidation is not performed in the secondary refining equipment, for example, when Ti deoxidation is performed at the time of tapping from a converter, Al addition is also performed immediately thereafter. Even if Al is not introduced immediately after Ti deoxidation, the amount of reduction of Ti oxide is not so large as long as it is within 5 minutes, so that it is preferable to be within 5 minutes. In the claims and the detailed description of the invention, "after adding Ti and deoxidizing or after deoxidizing Ti" means after the charged Ti is uniformly mixed in the molten steel. The addition of Ca is the same as the addition of Al, and it is desirable to add Ca within a short time after the addition of Al.

Next, the inventors have found that sol. Considering increasing the upper limit of the amount of Al. In the above-described deoxidation experiments using various elements in various orders, once the amount of dissolved oxygen is sufficiently reduced by using a strong deoxidizing element, oxides are formed even if a weak deoxidizing element is added. It was found that it hardly affected the state (size, number). That is, as described above, in order to finely disperse a large number of oxides, after gradually adding a strong deoxidizing element in the order of Ti, Al, and Ca, a deoxidizing element weaker than Ca, for example, Al,
Even if Ti, Si, Mn, Mg, and the like are added, most of the amounts are dissolved in steel without being involved in oxide formation.

Taking advantage of this effect, Ti, Al,
After deoxidation in the order of Ca, by adding Al, it is possible to add only the necessary composition of Al, which is effective for versatility of the welding material, in a state where the oxide is finely dispersed.

As described above, in order to control the composition, number and size of oxides to predetermined conditions, a deoxidizing method in a steel making process is important. As a suitable deoxidation method, the dissolved oxygen concentration before the deoxidation treatment after the steel baking from the converter is 20 to 80 p.
pm, and in a secondary refining process such as RH, Ti is added to a predetermined component value of 0.005 to 0.020% in a molten steel adjusted to have a predetermined component value of 0.005 to 0.020%, followed by deoxidation. Similarly, in a secondary process such as RH, first, sol. After adding Al having an Al content of 0.004 to 0.020% and further adding Ca, the final component (for sol. Al:
(0.005 to 0.09%), and the other components are added in an insufficient amount to adjust the final components.

The effect of oxides is not affected by the process of producing steel materials, such as normal rolling, controlled rolling, a combination of controlled rolling and tempering, and a combination of quenching and tempering. .

Next, the reasons for limiting the range of the basic components of the present invention will be described.

C is an effective component for improving the strength of steel, with the lower limit being 0.03%, and an excessive addition exceeding 0.18% remarkably reduces the weldability and HAZ toughness of the steel material. Was set to 0.18%.

Si is a component necessary for securing the strength of the base material and performing preliminary deoxidation, but the upper limit is set to 0.5% in order to prevent the toughness from being reduced by the hardening of the HAZ.

Mn is 0.4% as a component for ensuring the strength and toughness of the base material and for forming transformation nuclei of intragranular ferrite.
Although the above addition is necessary, the upper limit is set to 2.0% within an allowable range such as the toughness and cracking property of the welded portion.

The smaller the content of P is, the more desirable it is. However, in order to industrially reduce the content of P, a large cost is required. Therefore, the upper limit is set to 0.02%.

S is 0.00M as an element for producing MnS.
Although 1% is necessary, the upper limit is set to 0.01% within an allowable range such as the toughness and cracking property of the welded portion, and the upper limit is preferably 0.005%.

Sol. Al has a lower limit of 0.005% in order to increase the number of oxides and to suppress a decrease in toughness of the weld metal. FIG. The relationship between the Al content and the weld metal toughness is shown. Further, when Al is present in a large amount, all oxides become alumina and Ti-Al-Ca
Since no oxide is generated, the upper limit was made 0.09%.

Ti is added in an amount of 0.005% or more to form a Ti—Al—Ca oxide and a Ti nitride. However, when the amount of solute Ti increases, the HAZ toughness decreases. Therefore, the upper limit is set to 0.02%.

Ca must be added in an amount of 0.0005% or more in order to form a Ti—Al—Ca oxide. However, excessive addition leads to lowering of the melting point and coarsening of the oxide, and also impedes precipitation of MnS, and as a result reduces the intragranular ferrite structure, so the upper limit is 0.004%.

N is an extremely important element for the precipitation of Ti nitride. If the content is less than 0.001%, the amount of Ti nitride deposited is insufficient, and a sufficient amount of ferrite structure cannot be obtained.
The upper limit of 0.006 is set because an increase in solid solution N causes a decrease in HAZ toughness.

Although Cu is effective for improving the strength of the steel material, if it exceeds 1.0%, the HAZ toughness is reduced. Therefore, the upper limit is set to 1.0%.

Although Ni is effective for improving the strength and toughness of the steel material, the upper limit is 1.5% because an increase in the amount of Ni increases the production cost.

Nb is an element effective for improving the strength and toughness of the base material by improving the hardenability, but in the HAZ portion, excessive addition significantly lowers the toughness, so the upper limit is 0.03%. And

Since V, Cr and Mo have the same effect as Nb, they are 0.1%, 0.6% and 0.6%, respectively.
The upper limit was 0.6%.

B is a grain boundary ferrite harmful to HAZ toughness,
0.0002% or more from the suppression of the growth of the ferrite side plate and the fixation of the solute N in the HAZ by the precipitation of BN.
002% or less.

[0043]

EXAMPLE A 50 kg steel was prototyped with the chemical composition shown in Table 1. 1 to 9 are steels of the present invention, and 10 to 18 are comparative steels.
The prototype steel is melted from a converter and deoxidized at RH during vacuum degassing. Before the introduction of Ti, the dissolved oxygen of the molten steel is adjusted with Si, and thereafter, deoxidation described in Table 2 described below is performed. After casting into a 280 mm thick slab by continuous casting, it is heated and rolled to form a steel sheet having a thickness of 45 mm. Manufactured. The obtained steel sheet was subjected to one-pass SEGARC welding using a general-purpose welding material. The heat input is about 200 kJ / cm ² .

[0044]

[Table 1]

Table 2 shows the amount of dissolved oxygen before deoxidation, the order of deoxidation, the initial amount of Al (before adding Ca), the composition of the oxide, and the number of particles. Table 3 shows the rolling conditions, base metal properties, and HA of the steel sheet.
It shows Z toughness and weld metal (WM) toughness. The Charpy value for toughness evaluation was HAZ1m from the fusion line.
Nine tests were performed at the site of m and at the center of the WM, and the average value is shown.

[0046]

[Table 2]

[0047]

[Table 3]

As is clear from Table 3, the steels of the present invention Nos. 1 to 9 have excellent HAZ toughness as compared with the comparative steels. That is, when the particle size is 0.005 to 2.0 μm and T
The number of particles of the i-Al-Ca oxide is 100 to 3000 /
mm ² , and the absorbed energy of both HAZ toughness and WM toughness at −20 ° C. is as excellent as 50 J or more.

On the other hand, Comparative Examples 10 to 18 exhibited low toughness with a HAZ or WM of less than 40 J at -20 ° C in the Charpy test. These causes 10 because the initial dissolved oxygen amount did not reach the predetermined amount of the present invention,
11 is because the dissolved oxygen amount exceeded a predetermined amount, 12 was because the intermediate Al amount was less than the predetermined amount, and 13 was because the intermediate Al amount exceeded the predetermined amount. 14 and 15 are T
Since the order of adding i, Al, and Ca was different from that of the present invention,
16 was because the final amount of Al was low without the second addition of Al, 17 was because the Ca amount exceeded a predetermined amount, and 18 was C
This is because the amount a has fallen below a predetermined amount.

[0050]

According to the present invention, a steel sheet which satisfies the strict toughness requirements for destruction of ships, marine structures, middle and high-rise buildings, etc. is provided, and the effect brought to this kind of industrial field is extremely large. The contribution to society is very large in terms of safety.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the oxide number density and the Charpy absorbed energy of a heat affected zone of the welding.

FIG. It is a figure which shows the relationship between Al amount and the weld metal Charpy absorption energy.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C22C 38/00 301 C22C 38/00 301B 38/14 38/14 38/58 38/58 (72) Inventor Koji Ishida 1 Nishinosh 大, Oita City Nippon Steel Corporation Oita Works F term (reference) 4K013 AA09 BA08 BA14 CE01 CF13 DA03 DA08 DA12 EA18 EA19 EA25 EA28 FA02

Claims (2)

[Claims] Claims 1. A molten steel having a dissolved oxygen concentration of 20 to 80 ppm, to which Ti is added and deoxidized. A
Al is added so that 1 becomes 0.004 to 0.02%, then Ca is added, and then Al is further added, and C: 0.03 to 0.18%, : ≦ 0.5
%, Mn: 0.4 to 2.0%, P: ≦ 0.02%, S:
0.001 to 0.01%, sol. Al: 0.005 to
0.09%, Ti: 0.005 to 0.02%, Ca:
0.0005 to 0.004%, N: 0.001 to 0.0
And the balance is Fe and unavoidable impurities. The molten steel is cast in a continuous casting process, and has a particle diameter of 0.005 to 2.0 μm and a composition of Ca, Ti, and A.
1 to 100 to 30 complex oxides containing any two or more of
A method for producing a steel material having excellent toughness in a welded joint portion, wherein the steel material contains 0.0000 / mm ² . 2. In mass%, Cu: ≦ 1.0%, Ni: ≦
1.5%, Nb: ≦ 0.03%, V: ≦ 0.1%, C
r: ≦ 0.6%, Mo: ≦ 0.6%, B: 0.0002
The method for producing a steel material having excellent toughness in a welded joint portion according to claim 1, wherein the steel material contains one or more of 0.002% to 0.002%.